1. Field of the Invention
The present invention relates in general to automotive transmissions, and more particularly to transmissions of a continuously variable traction roller type.
2. Description of the Prior Art
Hitherto, various transmissions have been proposed and put into practical use particularly in the field of motor vehicles. One of them is a so-called "continuously variable traction roller transmission" which has motion transmitting traction rollers disposed between, and in engagement with, opposite toroidal input and toroidal output discs. Under operation of the transmission, the torque applied to the toroidal input disc from a prime mover (for example, an automotive engine) is transmitted through the traction rollers to the toroidal output disc.
During this, the rotation speed change ratio between the toroidal input and output discs is infinitely varied depending on the angle at which each traction roller inclines with respect to a common axis on which the input and output discs are arranged.
Japanese Pat. First provisional Publication No. 62-283256 shows one of the transmissions of the above-mentioned type. In the transmission of this publication, each traction roller is pivotally held, through an eccentric shaft, by a roller supporting member, so that an axial movement of the roller supporting member induces inclination of the traction roller.
The axial movement of the roller supporting member is effected by a hydraulic actuator to which an hydraulic pressure from a change speed control valve is applied. That is, the moved distance of the roller supporting member (viz., the inclination angle of the traction roller) is controlled by the hydraulic pressure applied to the hydraulic actuator.
The control valve comprises a sleeve whose axial movement is controlled by a speed change instruction means, such as a step motor or the like, and a spool which is mutually slidably received in the sleeve and axially moved in response to a rotational movement of the roller supporting member. When the sleeve and the spool are in their mutually neutral positions, the roller supporting member assumes its neutral position wherein the rotation speed change ratio between the input and output discs is 1 : 1. When the sleeve and the spool are moved away from the neutral positions in one or the other direction, the hydraulic pressure applied to the hydraulic actuator is increased thereby to incline the traction roller in a direction to increase or decrease the rotation speed change ratio.
A process cam is employed for converting the rotational movement of the roller supporting member to an axial movement of the spool against the above-mentioned movement of the sleeve, so that the spool is actuated in a direction to decrease the above-mentioned increased hydraulic pressure. That is, following the movement of the sleeve, the spool is moved against the movement of the sleeve, so that consequently the inclination angle of the traction roller is determined by an instruction value issued from the speed change instruction means, viz., step motor.
However, the above-mentioned transmission has the following drawbacks due to the inherent structure of the process cam employed therein.
That is, the cam surface of the process cam is shaped flat, so that the moved distance of the spool of the control valve is in proportion to the rotational movement of the roller supporting member, viz., the inclination angle of the traction roller. Accordingly, the increasing or decreasing rate of hydraulic pressure from the control valve is kept constant throughout the rotational movement of the roller supporting member, so that, as is shown in FIG. 8, the inclination angle 6 of the traction roller relative to the instruction value from the speed change instruction means (that is, relative to the number of steps of the step motor) becomes constant. Accordingly, as is understood from the graph of FIG. 9, the rotation speed change ratio has such a characteristic that under a speed increasing condition, the change rate of the speed change ratio is small, while under a speed decreasing condition, the same is large. This induces however that when the step motor is operated to run at a constant speed throughout the entire steps, the change of engine speed and that of the speed change ratio become ill-matched particularly at the time when the vehicle is being accelerated just after starting or subjected to a kickdown operation. The ill-matching makes the driver feel uncomfortable.
That is, under the vehicle being accelerated just after starting, the engine speed is greatly changed when, in response to a speed change operation of the transmission, the speed change operation is transferred from a speed reducing mode to a speed increasing mode. However, instantly, the engine speed change rate is reduced. Thus, underoutput of the engine is felt by the driver.
Furthermore, when, upon the vehicle being subjected to a kickdown, the speed change operation is transferred from a speed increasing mode to a speed reducing mode, the change in engine speed is small at an initial stage because the reducing rate of the speed change ratio is small, but tho change in the engine speed becomes great suddenly thereafter because the speed change becomes quick thereafter. This causes generation of an undesired acceleration shock of the transmission.